Electrical power transmission systems which distribute power from a number of different power sources typically use power buses to connect the elements of an electric power network to the network's nodes. In any such system, faults, or short circuits, can occur and it is important to isolate the portion of the system in which the fault is occurring so that the power distribution system can continue to operate in spite of the failure of one portion of the system, and so that the rest of the system is not damaged.
Faults generally cause a portion of the power bus to be grounded or to suffer leakage between phases. These faults are generally detected by measuring the imbalance, if any, between the amount of current entering and the amount of current leaving a specified portion of the power bus.
The object of the present invention is to provide an improved system and method for determining if a fault is occurring within a specified zone of a power bus.
The primary problem addressed by the present invention is distinguishing between faults which occur within a specified zone of a power bus and faults which occur outside the specified bus zone.
FIG. 1 shows the basic setup used both in the prior art and in the present invention for detecting faults in a power bus. This Figure shows one phase of a three phase power bus 20, with a specified bus protection zone 22. The power bus protection zone 22 receives and distributes power through feed lines F1 to FN. As shown in this particular example, all the feed lines except FN are used to receive power from remote power generators (not shown in FIG. 1), and feed line FN is used to transmit the power from these remote sources to the rest of the power network. In any case, all the current which enters and leaves the power bus protection zone 22 should, if there are no faults, flow through these feed lines F1 to FN.
The protection zone 22 is bounded by current transformers CT1 to CTN on the feed lines F1 to FN. Therefore points 36 and 38 shown in FIG. 1 are within the protection zone 22, while point 39 is outside the protection zone. Were a fault to occur at point 36 or 38, the fault would be an internal fault (i.e., within the bus protection zone 22), while a fault at point 39 would be an external fault.
Internal faults are detected by measuring the net current (generally called the differential current) I.sub.dif flowing into the bus protection zone 22. To do this, a current transformer CTx is used on each phase of each feed line Fx to develop a current signal (called the secondary current) proportional to the current flowing through that phase of that feed line into the bus protection zone 22. In most applications, these current signals are summed simply by attaching all the current transformers for each phase in parallel with an current measuring device 24. The current measuring device 24 generates an output indicator--typically a voltage signal proportional to the differential current I.sub.dif.
In other applications, the current signal from each current transformer can be individually measured by a current based analog to digital converter, and the resulting digital signals can be summed and otherwise processed by a standard digital computer.
A fault detector 30, which can be an electronic circuit or a programmed digital computer, analyzes the differential current signal I.sub.dif to determine if there is a fault in the bus protection zone 22. When the fault detector 30 detects an internal fault it generates a trip signal on trip line 32.
Each feed line Fx is coupled to the power bus 20 by a circuit breaker CBx which disconnects the feed line from the power bus 20 if fault detector 30 detects an internal fault and generates a trip signal on trip line 32.
In theory, if there are no faults in the bus protection zone, the differential current I.sub.dif should be equal to zero. On the other hand, if there are faults in the bus protection zone 22, such as at point 36 or 38, the amount of current entering the bus protection zone 22 will not be equal to the current leaving it, and a nonzero I.sub.dif will be generated.
Therefore it would appear to one not experienced in the design of power buses that if I.sub.dif remains nonzero for even a small period of time then there must be a fault in the bus protection zone.
The problem with this method of fault detection is that an external fault, such as a fault at point 39, can cause the iron core of the current transformer nearest the fault, or even other ones of the current transformers, to saturate. When a current transformer saturates, the signal on its secondary is no longer proportional to the current flowing through its primary coil, and thus a differential current I.sub.dif will be developed in the secondary circuit even though the fault is outside the protection zone.
A number of different solutions to this problem (i.e., the current transformer saturation caused by external faults) have been used in prior art systems. Historically, the solution longest in use is the use of a percentage-differential relay. The percentage-differential relay compares the differential current I.sub.dif with a restraint signal that is based on the sum of the magnitudes of currents flowing in all of the feed lines. For instance, the relay might require that the differential current exceed five percent of the total current flowing into the power bus to trip the bus's circuit breakers. The percentage-differential relay has the disadvantages that it requires measurement or computation of the individual feed line currents for comparison to the differential current, that low-current internal faults may be masked and therefore not detected, and that heavy external faults can also produce inadequate restraint signals to block tripping (e.g., if the adjacent current transformer saturates very badly).
Another prior art technique is the use of a high impedance relay. This technique makes use of the fact that when a current transformer saturates, the secondary exciting impedance of the transformer is reduced. The current transformers on the bus's feed lines are coupled in parallel across a varistor burden with a voltage measuring element. If a current transformer saturates, the varistor burden forces false differential current through the saturated transformer (which has a reduced impedance). This prevents the varistor voltage from becoming large enough to rise above a preselected trip level. While this technique is effective, it requires current transformers which are physically wired together, and the signals generated by current transformers cannot be used for other purposes. Also, dangerous voltages may develop on the current transformer's secondary wiring during internal faults.
A third prior art technique is to use a feed line current sensor which cannot saturate. The primary example of this technique is the use of a linear coupler relay. In this relay, the secondaries of special air-core current transformers, which cannot saturate and which generate a voltage proportional to the current in the transformer's primary, are connected in series with a simple voltage-sensing element. The liner coupler relay is simple, fast and effective, but the linear couplers are also expensive and not useable for other current measurement purposes.
Referring to FIGS. 2A and 2B, the present invention is based on the observation that the differential current I.sub.dif is generally sinusoidal in shape for internal faults and substantially nonsinusoidal in shape, if present at all, for external faults. In FIG. 2A the solid line waveform is the secondary current I.sub.S of an unsaturated current transformer, while the dashed waveform is the secondary current I.sub.S of a saturated current transformer. As indicated, while a saturated current transformer does not track the primary (sinusoidal) current very well, it does recover and reproduce the primary current wave for at least a small portion of each cycle. FIG. 2B shows the differential current waveform I.sub.dif generated by the setup in FIG. 1 when an external fault occurs at 39 and one of the current transformers CTx is saturated as shown in FIG. 2A.
A related observation is that the differential current I.sub.dif is nonzero a much greater percentage of the time for internal faults than for external faults. In particular, the present invention solves the problem of distinguishing internal faults from external faults by looking at the percentage of time that the differential current is nonzero (actually, the amount of time that it is above a fault threshold value). For instance, looking at FIG. 2B, it is clear that a sinusoidal waveform is nonzero a much greater percentage of the time than the differential current waveform shown in FIG. 2B for a system with a saturated current transformer. The advantages of this approach are that it permits the use of standard current transformers, it is simple to implement, and permits the signals from the current transformers to be used for other measurement functions.
It is therefore a primary object of the present invention to provide an improved technique for distinguishing between faults inside a defined bus protection zone and faults outside that zone.
Another object of the present invention is to provide a technique for detecting internal power bus faults by determining whether the net current entering the power bus exceeds a fault threshold at least a specified percentage of the time.